Cardiac Output Monitoring – Invasive and Noninvasive

Authors, Journal, Affiliations, Type, DOI

• Virendra K. Arya, Waiel Al-Moustadi, Vikas Dutta
• Current Opinion in Critical Care, 2022; 28(3):340–347
• Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba, Canada
• Review article (narrative)
• DOI: 10.1097/MCC.0000000000000937

Overview

This 2022 narrative review surveys CO monitoring technologies across three invasiveness tiers — invasive, minimally invasive, and noninvasive — and their clinical utility in critically ill patients. PAC thermodilution remains the gold standard and is experiencing a revival in cardiac ICUs, especially for RV failure, PAH, and mixed shock. Critical care echocardiography (CCE/TTE) is the only noninvasive modality with clinically acceptable agreement with thermodilution (±25% error) and adds differential diagnostic value. All other noninvasive methods show unacceptable accuracy in critically ill patients and cannot be recommended for routine use. Practitioner experience and technology availability remain the primary determinants of CO monitor selection.

Keywords

cardiac output, critical care echocardiography, invasive monitoring, noninvasive monitoring, pulmonary artery catheter

Key Takeaways

Introduction

• CO monitoring is essential in critically ill patients, especially mixed shock — clinical signs (capillary refill) and laboratory parameters (lactate, SvO₂) may not differentiate shock types
• Three tiers: invasive, minimally invasive, noninvasive

Invasive Modalities

Pulmonary Artery Catheter Thermodilution

• PAC introduced in the 1970s; popularity declined due to outcome studies in medical ICUs failing to show survival benefit and reported complications
• PAC revival: revisited utility in RV failure, PAH, LV failure, and mixed shock
• Modern PAC enhancements: RV port (simultaneous PA and RV waveform display), continuous CO monitoring (CCOM), SvO₂ sensor at tip, temporary pacing capability
• RV decompensation indicator: overlap of RV end-diastolic pressure over PA end-diastolic pressure is the earliest sign of RV decompensation
• CCOM algorithm: continuously monitors CO, SV, SvO₂, SVR, and RVEF every 20s via a temperature filament (avoids volume overload of cold saline); uses heat content to blood flow detected by rapid response thermistor
• PAC preferred in cardiac surgical ICU for differentiating mixed shock
• Key pitfall: misinterpretation of data is the leading cause of PAC-related complications; training essential
• PAC CO measured in RV — not a surrogate for LV CO in intracardiac shunts or tricuspid valve abnormalities
• Intermittent CO errors: injectate temperature variability, volume errors, respiratory cycle timing
• Contraindications to wedging: PAH and severe MR (PA rupture risk); severe MR makes wedged waveform resemble PA waveform

Transpulmonary Thermodilution (TPT)

• Introduced early 2000s (PiCCOplus, EV1000/VolumeView) as less invasive PAC alternative
• Measures left heart CO (vs PAC measuring right heart CO)
• Requires femoral/axillary/brachial arterial cannula + central venous line (no PAC)
• Additional parameters: ITBV, GEDV, EVLW, PVPI
• Correlates well with PAC-derived CO; can track changes as low as 12% with ≥3 cold boluses
• Unreliable in: severe valvular regurgitation, therapeutic hypothermia, circuit leaks, ECMO, low CO (<2 L/min), pulmonary edema (indicator recirculation)
• Technical limitation: crosstalk phenomenon if central line and arterial cannula are on same femoral side
• Main limitation: intermittent measurement — cannot detect short-term hemodynamic changes

Minimally Invasive

Arterial Pulse Analysis

LidCOplus (Pulse Power Analysis + Lithium Dilution)

• Continuous CO by pulse power analysis (PulseCO algorithm); calibrated by lithium dilution
• Lithium injected peripherally or centrally; analyzed by ion-selective electrode on arterial line
• Correlates well with PAC thermodilution; considerably less invasive
• Contraindications: weight <40 kg, first trimester pregnancy, coronary artery bypass grafting with aortic clamping, lithium therapy, renal failure; high-dose nondepolarizing neuromuscular blockers → CO overestimation

Pulse Wave Analysis (PWA) — PiCCO, VolumeView, FloTrac

• Two types: externally calibrated (VolumeView, PiCCO — use TPT for calibration) and non-calibrated (FloTrac — calibrated by another method e.g. echocardiography)
• Produce similar CO to PAC in stable rhythm and respiratory pattern
• Fail in dynamic situations requiring significant inotropic or vasopressor support

Esophageal Doppler Monitoring (EDM)

• Ultrasound probe in esophagus measures VTI and stroke distance; CO = VTI × aortic CSA × HR
• CSA from ultrasound M-mode or nomogram (age, height, weight)
• NICE guidelines support EDM in complex/high-risk surgical procedures to reduce complications, hospital stay, and central line use
• Limitations: operator-dependent variability 10–12%; errors in aortic CSA/nomogram; fixed proximal-to-distal aortic flow assumption invalid in hemodynamic instability; misalignment >20° causes poor accuracy

Noninvasive Modalities

Pulse Wave Analysis (Noninvasive)

• Volume clamp method (ClearSight, CNAP — finger cuff): inaccurate in severe peripheral edema, hypothermia, circulatory shock, severe vasoconstriction, high-dose vasopressors
• Meta-analysis: finger cuff noninvasive CO not interchangeable with invasive reference methods in adult surgical/critically ill patients
• Applanation tonometry (T-Line, DMP-Life): motion artifact → false readings; limited validation
• Not recommended in critically ill patients with shock (will have arterial catheter anyway)

Thoracic Bioimpedance and Bioreactance

• Bioimpedance: assumes thoracic impedance changes proportional to SV; poor accuracy in intubated cardiac surgical patients vs continuous thermodilution
• Meta-analysis: modest agreement and inadequate percentage error for bioimpedance — not interchangeable with bolus thermodilution
• Electrical interference and increased lung water render these devices ineffective in ICU
• Bioreactance: measures phase shifts from pulsatile flow (less susceptible to thoracic fluid artifact)
• NICOM study (cardiogenic shock): poor correlation of bioreactance compared to both Fick and PAC thermodilution — likely due to thoracic fluid overload and low-flow state

Pulse Wave Transit Time (esCCO)

• Uses R-wave to pulse oximeter waveform rise-point interval
• esCCO: poor accuracy and precision vs invasive TPT in cardiac surgery and liver transplant patients
• Unable to track CO changes induced by preload increase or vasomotor tone variations in ICU

Partial CO₂ Rebreathing (NICO, INNOCOR)

• Modified Fick principle using expired CO₂; percentage error 40–49% (validation study + meta-analysis)
• Not suitable for ICU clinical use
• Influenced by non-steady respiratory state, unsealed airway, intrapulmonary shunting, atelectasis, severe gas exchange abnormalities

Transthoracic Doppler (USCOM)

• Suprasternal (trans-aortic, left heart CO) or left sternal edge (trans-pulmonary, right heart CO)
• Meta-analysis of 10 studies: pooled weighted percentage error 42.7% vs bolus thermodilution (threshold for acceptability ≤30%)
• Does not achieve acceptable precision vs thermodilution

Critical Care Echocardiography (CCE)

• CCE can be both diagnostic tool and CO monitor; rapidly becoming an essential bedside ICU tool
• TTE most used; TEE useful in intubated/ventilated patients
• TTE vs PAC: percentage error 25% — within acceptable range for CO estimation; significantly correlated with PAC
• Severely ill pregnant women: TTE showed excellent agreement with PAC
• Noncardiologist ICU physicians can achieve accurate CO assessment with focused Doppler/VTI training added to basic TTE
• CCE complementary to PAC (not exclusive) in septic shock requiring advanced hemodynamic monitoring
• CCE as future reference: given noninvasive nature and accuracy, TTE could serve as reference for validating other CO techniques
• Certification: American Society of Echocardiography (ASE); ESICM published core critical care ultrasound competencies
• Barrier: lack of formal training programs and clearly defined competencies in most countries

Limitations of the Document

• Narrative review — no systematic methodology, no meta-analysis
• No original data; opinion-based device comparisons without formal statistical pooling
• Device-specific studies cited may reflect different software versions, patient populations, and clinical settings
• No head-to-head comparison table with formal percentage error statistics for all modalities
• Predominantly anesthesia/perioperative literature cited; some ICU-specific evidence limited

Key Concepts Mentioned

• concepts/Cardiac-Output-Measurement — central concept; all devices reviewed against PAC thermodilution standard
• concepts/Invasive-Hemodynamic-Monitoring-CS — PAC revival in cardiac ICUs; mixed shock differentiation
• concepts/Right-Heart-Catheterization — PAC as gold standard; waveform interpretation pitfalls
• concepts/Right-Ventricular-Failure — RV port overlap waveform as earliest indicator of RV decompensation
• concepts/ECPELLA — ECMO as contraindication for TPT and pulse contour analysis

Key Entities Mentioned

• entities/Pulmonary-Embolism — PAH context for PAC wedging contraindication

Wiki Pages Updated

• wiki/sources/co-cocc-2022.md — created
• wiki/concepts/Cardiac-Output-Measurement.md — updated with CCOM, CCE/TTE data, NICOM, esCCO, PAC revival section
• wiki/sourceindex.md — updated
• wiki/wikiindex.md — updated